(1) Field of the Invention
The present invention relates to non-destructive testing and particularly to the non-invasive examination of soft tissue and body organs. More specifically, this invention is directed to medical ultrasonic equipment and particularly to pulse-echo body scanners. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited for use in diagnostic medicine. Apparatus and techniques which permit the non-invasive examination of soft tissue organs are, for obvious reasons, of considerable interest. Presently available techniques for performing "imaging" of soft tissue organs include x-ray, nuclear medicine, thermography and, to a much lesser extent, diagnostic ultrasound. Nuclear medicine is, of course, an invasive technique, thermography has very limited utility and the degree of information which can be provided by conventional x-rays is limited; i.e., x-rays are not well suited for the imaging of soft tissues. Further, with imaging techniques other than diagnostic ultrasound, there may be some restriction to repeating the test if inconclusive results are obtained. In the case of nuclear medicine, for example, an inconclusive or unsatisfactory radioisotope scan may require the patient to be subjected to the reinjection of the radioisotope. As an additional disadvantage thereto, radioisotope scans and x-rays are notoriously expensive procedures.
Ultrasonic diagnostic techniques, because of the very high benefit to risk ratios for the patient and the ability to perform imaging of soft tissue organs that no other modality can provide, are attracting ever increasing interest. Thus, ultrasonic diagnosis has found applicability in obstetrics and gynecology, cardiology, neurology, ophthalmology and urology in addition to crossing over medical disciplines with the imaging of various internal body organs. In some situations invasive techniques for studying the heart, such as cardiac catheterization and angiography, can be replaced by ultrasonic techniques. Similarly, ultrasonic diagnosis has found use in the diagnosis of mitral stenosis. The widespread utility notwithstanding, the adoption of this modality has been impeded by inherent limitations in the equipment previously available.
Ultrasonic diagnostic instruments operate on either a pulse-echo or Doppler principle. The pulse-echo principle, which is primarily used for the imaging of soft body tissue, involves the transmitting of short bursts of ultrasonic energy and recording echoes reflected from anatomic structures within the body. Since the time required for an emitted pulse to return as an echo indicates the distance of the target structure from the transducer, the "echo gram" provides both a picture of the object and a graphic recording of any changes in the objects position. Thus, ultrasonic diagnosis is based on the reflection of ultrasonic waves which occur at the boundaries between different tissues within the body. A fraction of the incident energy is reflected if there is a change in characteristic impedance at such a boundary; impedance being defined as the product of the density of the tissue multiplied by the velocity of sound. Although the echoes which correspond to soft tissue boundaries have very small amplitudes, these echoes can be detected by a receiver having the requisite sensitivity. Energy which is not reflected travels beyond the boundary, and may be reflected at deeper boundaries. The maximum penetration is limited by the attenuation of the ultrasonic wave in passing through the tissues; attenuation being defined as the decrease in intensity of the sound pulse per unit of distance as it propagates in the medium and loses energy as the result of absorption and scattering.
Ultrasonic diagnostic instruments employ a transducer which converts electrical signals into acoustic pulses which are coupled into the tissue of the patient. The transducer may also serve the dual function of receiver for detecting the reflected pulses from within the patient. The transducers employed in ultrasonic body scanners are typically piezoelectric elements comprised of ceramic materials such as synthetic lead zirconate titanate. An ultrasonic diagnostic instrument will also comprise an oscillator which establishes the pulse repetition frequency and a linear power amplifier which excites the transducer through a coupling circuit. A decoupler permits the transducer to be used as both a transmitter and a receiver. The received pulses; i.e., the echoes returned from within the patient's body; are converted into electrical signals in the manner known in the art, these electrical signals are processed and the processed signals are presented on a display. The display will typically be a cathode ray tube and the oscillator which controls the transducer may also be employed to generate a time base trace for the display.
In order to obtain maximum utility from the instrument, two-dimensional images of various organs or body regions of interest must be generated. This can be accomplished by "scanning " wherein the transducer is moved back and forth. In the prior art the most common method of scanning involves contact scanning in which the transducer is placed directly on the patient's skin and moved, through a type of compound scan, in stepwise fashion. The information obtained must be optimized through coordinated movement of the transducer to achieve a meaningful image. Accordingly, a high degree of skillful operator interaction with the instrument is essential for a successful ultrasonic examination employing prior art equipment and it has been exceedingly difficult to duplicate initial test results since repeatability was almost totally dependent upon operator placement of the transducer.
The high degree of operator skill required and the extreme difficulty in repeating test results have, in part, been a consequence of the use of small size contact transducers; this small size resulting from the necessity of fitting the transducer to the contour of the skin. Prior art ultrasonic body scanners, as a consequence of their use of small contact transducers, were also characterized by slowness of use since the ability to find the area of interest was limited to trial and error scans. It is to be noted that the small size of the transducers, the slowness of the procedure and the difficulty in obtaining repeatability was also attributable to the fact that the prior transducers and associated apparatus lacked both the ability to electronically focus the "beam" of ultrasonic energy over the entire examination depth of interest and the ability to easily aim the "beam".
Prior ultrasonic diagnostic equipment has also been characterized by insufficient resolution over the desired examination range in the body; this examination range or field of examination typically being on the order of 20 centimeters. In order to be practical, an ultrasonic diagnostic device must have the ability of providing real time images of high resolution. The required characteristics, which result only from minimizing beam width and side lobes, have been lacking in the prior art. A further deficiency of prior art ultrasonic body scanners has resided in their poor dynamic range. The returns or echoes from the signal propagated into the body may vary over a range of 100 db. It is impossible to record the 10.sup.6 shades of gray which correspond to a 100 db range. It is, accordingly, prior practice to compress the signals generated by echoes through either the use of logarithmic amplifiers or by simple time gain compensation circuits. Using the prior art compression techniques, however, important information contained in the received signals has been lost.
With particular respect to the transducers employed in the prior art, as briefly noted above the transducers previously used have not been capable of being focused electronically to achieve variable examination depth. Prior art transducers have typically been of a flat contour; i.e., have had no natural focus; and accordingly have been characterized by large magnitude side lobes which give spurious signals from off-axis targets and also result in ambiguity in range measurements because of the different path lengths for the echoes from the same object.